Move insertlane to ISLE (#3544)

This also fixes a bug where `movsd` was incorrectly used with a memory
operand for `insertlane`, causing it to actually zero the upper bits
instead of preserving them.

Note that the insertlane logic still exists in `lower.rs` because it's
used as a helper for a few other instruction lowerings which aren't
migrated to ISLE yet. This commit also adds a helper in ISLE itself for
those other lowerings to use when they get implemented.

Closes #3216

cranelift/codegen/src/machinst/isle.rs

@@ -175,6 +175,10 @@ macro_rules! isle_prelude_methods {
         fn u64_from_ieee64(&mut self, val: Ieee64) -> u64 {
             val.bits()
         }
+
+        fn u8_from_uimm8(&mut self, val: Uimm8) -> u8 {
+            val
+        }
     };
 }